In the field of optical communication, communication systems are used in which optical signals are transmitted bidirectionally through a single optical fiber. In such a communication system, there are many cases where transmission and reception of optical signals are performed using a single-fiber bidirectional optical transmitter/receiver.
The single-fiber bidirectional optical transmitter/receiver has a single-fiber bidirectional optical transmission/reception device that is formed by a laser diode (LD) for optical transmission and a photo diode (PD) for optical reception being incorporated into a single housing (for example, refer to Patent Document 1).
FIG. 1 is a perspective view of an interior of a main housing of a single-fiber bidirectional transmitter/receiver. In many cases, the main housing 2 of the single-fiber bidirectional transmitter/receiver is formed of a steel plate of which surface is zinc-plated or nickel-plated. A single-fiber bidirectional optical transmission/reception device 4 and a circuit board 12 are accommodated in the main housing 2.
The single-fiber bibidirectional optical transmission/reception device 4 is a device including a laser diode (LD) for optical transmission and a photo diode (PD) for optical reception that are incorporated into a single housing to be integrated with each other. An avalanche photo diode (APD) is used in many cases as a photo diode (PD) for optical reception. The single-fiber bidirectional optical transmission/reception device 4 includes a connector part 6 to which an optical fiber is connected, an LD stem part 8 into which a laser diode is incorporated, and a PD stem part 10 into which a photo diode is incorporated.
An optical fiber (not shown in the figure) is connected to the connector part 6. An optical signal transmitted through the optical fiber is changed into an electric signal by the photo diode of the PD stem part 8, and is output from the PD stem part 8. A lead-terminal 8a for the photo diode extends from the PD stem part 8. The lead-terminal 8a is connected to an electrode terminal 12a of the circuit board 12. On the other hand, an electric signal for optical communication is changed into an optical signal by the laser diode of the LD stem part 10, and is transmitted to the optical fiber. A lead-terminal 10a for the laser diode extends from the LD stem part 10. The lead-terminal 10a is connected to an electrode terminal 12b of the circuit board 12.
Formed on the circuit board 12 are a drive circuit for driving the single-fiber bidirectional optical transmitter/receiver device 4 and a process circuit for processing electric signals supplied to the bidirectional optical transmitter/receiver device 4 and electric signals output from the single-fiber bidirectional optical transmission/reception device 4.
Patent Document 1: Japanese Laid-Open Patent Application No. 2006-294746
In the single-fiber bidirectional optical transmitter/receiver having the structure illustrated in FIG. 1, a voltage of an electric signal for driving the laser diode is a relatively high-voltage, which is about 1 V. Thus, a large drive current is supplied from the circuit board 12 to the LD stem part 10 through the lead-terminal 10a. On the other hand, a voltage of an electric signal output from the photo diode of the PD stem part 8 through the lead-terminal 8a is about several millivolts to several microvolts. Thus, an output current output from the PD stem part 8 through the lead-terminal 8a is also a very small current.
Because the lead-terminal 10a of the LD stem part 10 and the lead-terminal 8a of the PD stem part 8 are close to each other, it is possible that a cross-talk is generated electrically or electromagnetically between the lead-terminal 10a through which the large drive current flows and the lead-terminal 8a through which the very small current flows. If such a cross-talk occurs, a noise may be applied to the output signal output from the photo diode to the circuit board 12 through the lead-terminal 8a. 
Additionally, in the single-fiber bidirectional optical transmitter/receiver having the structure illustrated in FIG. 1, the metal-made main housing 2 and the housing of the single-fiber bidirectional optical transmission/reception device 4 are caused to be a ground potential by being electrically connected to a signal grounding part (signal-ground: SG) of the circuit board 12. However, if an electrostatic discharge (ESD) occurs to the metal-made main housing 2 of the single-fiber bidirectional optical transmitter/receiver, there may be a case where an electric discharge is applied to the SG of the circuit board 12. In such a case, a noise enters a signal processing circuit formed on the circuit board 12, which may generate an SNR error (signal/noise ratio error). Additionally, a large current generated by the electrostatic discharge (ESD) flows to the housing of the single-fiber bidirectional optical transmission/reception device 4, which may cause a damage of electronic parts in the single-fiber bidirectional optical transmission/reception device 4 due to destruction by electrostatic discharge.